To minimize the amount of emissions exhausted into the atmosphere, modern automotive vehicles generally include one or more catalytic converters, or emission control devices, in the exhaust system of the vehicle. These emission control devices store oxygen and NOx (collectively, “oxidants”) from the vehicle exhaust stream when the engine is operated with a relatively lean air/fuel ratio. On the other hand, when the engine is operated with a relatively rich air/fuel ratio, they release the stored oxygen and NOx, which then react with the HC and CO produced by the engine. In this way, the emission of both NOx and hydrocarbons (HC and CO) into the atmosphere is minimized.
The inventors have recognized a disadvantage with conventional air-fuel ratio control systems. In particular, the inventors have recognized that these systems attempt to maintain the engine at stoichiometry (or another desired air-fuel ratio). However, this has the disadvantage that engine air-fuel control is decoupled from the state of oxidant storage of the emission control device. The convention system relies on air-fuel feedback to compensate for this oversight.
To overcome disadvantages with prior approaches, the inventors have developed a method for controlling the engine air-fuel ratio to maintain the oxidant level stored in the emission system at a desired set-point level. However, the inventors have further recognized that there is another dimension to the emission system, namely the depth into catalyst along the direction of flow. I.e., oxidants are stored in different proportions depending on the depth, or length, of the catalyst brick. For example, at the front face of a catalyst brick, there is a high chance of oxidants being stored than at the back of the brick. As such, a control system that controls the engine based on the oxidants stored in the emission control device should consider where along the depth of the catalyst the set-point is chosen.